Download Cardiac muscle

NAME: OLADEJI OMOLOLA
MATRIC NUMBER: 14/MHS01/149
DEPARTMENT: MEDICINE AND SURGERY
COURSE: ANA 203
WRITE ON THE HISTOLOGY OF MUSCLE AS A TISSUE AND
STATE ITS TYPES
MUSCLE AS A TISSUE
Muscle tissue is categorized on the basis of a functional property: the ability of its cells to
contract. In muscle tissue, the bulk of the cytoplasmic volume consists of the contractile protein
fibrils actin and myosin. Muscle is responsible for movement of the body and changes in the
size and shape of internal organs. Muscle cells are generally referred to as muscle fibers.
Muscle fibers are typically arranged in parallel arrays, allowing them to work together
effectively. Thus the plasma membrane of muscle cells is sometimes called the sarcolemma and
their cytoplasm sarcoplasm. Their endoplasmic reticulum is called sarcoplasmic reticulum and
their mitochondria are sometimes called sarcosomes. The contractile fibers that lie in the
sarcoplasm are known as myofibrils and the embryonic precursors of skeletal muscle cells are
called myoblasts.
The entire muscle is surrounded by a connective tissue called the epimysium. The
muscle is made up of smaller bundles known as fascicles. Fascicles are actually bundles of
individual muscle cells (myofibers or myocytes). These bundles are surrounded by a connective
tissue sheath called the perimysium. Each fascicle is made up of several muscle cells known as
myocytes. They may also be called myofibers or muscle fibers. Each muscle cell is surrounded
by a connective tissue sheath known as the endomysium. This sheath is very important in the
physiology of muscle contraction because it electrically insulates the individual muscle cells
from each other. At the ends of the muscle all of the connective tissue sheaths (epimysium,
perimysium, and endomysium) converge to form a tendon which will connect the muscle to its
attachment site.
The thin myofilaments are composed mainly of a protein known as actin. Actin filaments
are anchored into the z- line of a sarcomere. The thick myofilaments are composed mainly of
the protein myosin. It is the orderly overlapping of the actin and myosin filaments that give
cardiac and skeletal muscle their striated appearance (light and dark bands). The A band is the
dark band and corresponds to the length of a bundle of myosin filaments. Because muscle
contraction is a sliding of the myofilaments past each other we do not see any of the
myofilaments actually shorten. However the width of the banding patterns change as the
degree of overlap changes. Because the A band corresponds to the length of the myosin
filaments, and these filaments do not shorten, the width of the A band also does not shorten.
The light bands are known as I bands. The I bands are composed mainly of actin filaments. Each
I band is bisected by a protein disc known as the Z-line.
Actin filaments are anchored into the Z- line. During muscle contraction the actin
filaments slide over the myosin filaments which results in a shortening of the I band. In the
middle of the A band is a somewhat lighter area known as the H zone. This zone corresponds to
the area where we have myosin not overlapped by actin (the area between the thin filaments).
During muscle contraction the actin sliding over the myosin encroaches into this area so that
the H zone shortens. In the middle of the H zone we see a dark band known as the M line. The
M line is comprised of protein fibers that function to anchor the myosin filaments. The area
between two Z lines is known as a sarcomere. The sarcomere is the functional or contractile
unit of muscle. Muscles contract when they receive a motor impulse from a motor nerve. These
nerve impulses serve only a limited number of muscle fibers. The muscle fibers served by a
single motor neuron make up a structure known as a motor unit. Motor units allow for selective
contraction of muscle fibers so that we may control the strength and extent of muscle
contraction. Without motor units a nerve impulse to the muscle would result in the entire
muscle contracting to its full extent.
The types of muscle:
Three types of muscle tissue can be identified histologically: skeletal muscle, cardiac muscle
and smooth muscle. The fibres of skeletal muscle and cardiac muscle exhibit cross striations at
the light microscope level and they are both referred to as striated muscle.
Skeletal muscle
Skeletal muscle constitutes the muscle that is attached to the skeleton and controls motor
movements and posture. There are a few instances where this type of muscle is restricted to
soft tissues: the tongue, pharynx, diaphragm and upper part of the esophagus. Skeletal muscle
fibers (cells) are actually a multinucleated syncytium formed by the fusion of individual small
muscle cells or myoblasts, during development. They are filled with longitudinally arrayed
subunits called myofibrils. The myofibrils are made up of the myofilaments myosin (thick
filaments) and actin (thin filaments). The striations reflect the arrangement of actin and myosin
filaments and support structures. The individual contractile units are called sarcomeres. A
myofibril consists of many sarcomeres arranged end to end. The entire muscle exhibits crossstriations because sarcomeres in adjacent myofibrils and muscle fibers are in register. The most
obvious feature in longitudinal sections of skeletal muscle is the alternating pattern of dark and
light bands, called respectively the A (anisotropic) and I (isotropic) band. The I band is bisected
by a dense zone called the Z line, to which the thin filaments of the I band are attached. The
nuclei are located peripherally, immediately under the plasma membrane (sarcolemma). The
thickness of individual muscle fibers varies (depending for example on location in the body and
exercise) but each fiber is of uniform thickness throughout its length. Skeletal muscle fibers do
not branch.
Connective tissue elements surround muscle fibers. Individual muscle fibers are
surrounded by a delicate layer of reticular fibers called the endomysium. Groups of fibers are
bundled into fascicles by a thicker CT layer called the perimysium. The collection of fascicles
that constitutes one muscle is surrounded by a sheath of dense CT called the epimysium, which
continues into the tendon. Blood vessels and nerves are found in the CT associated with muscle.
The endomysium contains only capillaries and the finest neuronal branches.
Cardiac muscle
Cardiac muscle makes up the muscular walls of the heart (myocardium). It is 'involuntary'
because its contractions are not under your control. Cardiac muscle is intrinsically contractile
but is regulated by autonomic and hormonal stimuli. Cardiac muscle exhibits striations because
it also has actin and myosin filaments arranged into sarcomeres. Generally these striations do
not appear as well-defined as in skeletal muscle. (At the ultrastructural level, some differences
in the arrangement of the sarcoplasmic retiuculum and T tubules can be seen. Cardiac muscle
also has a much greater number of mitochondria in its cytoplasm). At the light microscope level,
a number of features distinguish cardiac from skeletal muscle. Cardiac muscle cells have only
one or two nuclei, which are centrally located. The myofibrils separate to pass around the
nucleus, leaving a perinuclear clear area (not always evident in standard preparations). This
clear area is occupied by organelles, especially mitochondria. As in skeletal muscle, individual
muscle fibres are surrounded by delicate connective tissue. Numerous capillaries are found in
the connective tissue around cardiac muscle fibers.
Cardiac muscle cells are joined to one another in a linear array. The boundary between
two cells abutting one another is called an intercalated disc. Intercalated discs consist of several
types of cells junctions whose purpose is to facilitate the passage of an electrical impulse from
cell to cell and to keep the cells bound together during constant contractile activity. Unlike
skeletal muscle fibers, cardiac muscle fibers branch and anastomose with one another.
Although made up of individual fibers, heart muscle acts as a functional syncytium during
contraction for the efficient pumping of blood. Specialized fibers, called Purkinje fibers, arise
from the atrioventricular node and travel along the interventricular septum toward the apex of
the heart, sending branches into the ventricular tissue. Purkinje fibers are of larger diameter
than ordinary cardiac fibers, with fewer myofibrils and an extensive, well-defined clear area
around the nucleus. They conduct impulses at a rate about four times faster than that of
ordinary cardiac fibers and serve to coordinate the contraction of the atria and ventricles.
Smooth muscle
Smooth muscle is the intrinsic muscle of the internal organs and blood vessels. It is also
found in the iris and ciliary body of the eye and associated with hair follicles (arrector pili). No
striations are present in smooth muscle due to the different arrangement of actin and myosin
filaments. Like cardiac muscle, smooth muscle fibres are intrinsically contractile but responsive
to autonomic and hormonal stimuli. They are specialized for slow, prolonged contraction.
Smooth muscle fibres are generally arranged in bundles or sheets. Each fibre is fusiform in
shape with a thicker central portion and tapered at both ends. The single nucleus is located in
the central part of the fibre. Fibres do not branch. They range enormously in size, from 20 (in
wall of small blood vessels) to 500 (in wall of uterus during pregnancy) micrometers. Smooth
musclebfibres lie over one another in a staggered fashion (tapered part of one fibre over
thicker part of another).
In longitudinal sections, it is often not possible to distinguish the fibre boundaries, and
smooth muscle may closely resemble connective tissue (bundles of collagen). Where smooth
muscle bundles are interlaced with bundles of connective tissue (eg. in the uterus), one can
distinguish the smooth muscle by the lorientation of the nuclei (all oriented in the same
direction), and the greater abundance of nuclei per unit area (everynsmooth muscle cell has a
nucleus, fibroblast nuclei are more scattered in bundles of CT). Also, smooth muscle nuclei
often have a corkscrew shape in longitudinal section due to contracton of the muscle fibre
during fixation. In cross section, smooth muscle appears as profiles of various sizes, depending
on whether the cut went through the thick central part or tapered end of any individual fibre.
Nuclei are seen only in the thicker profiles. One distinguishing physiological feature of smooth
muscle is its ability to secrete connective tissue matrix. In the walls of blood vessels and the
uterus in particular, smooth muscle fibres secrete large amounts of collagen and elastin.